Tunable capacitors are widely employed in radio frequency (“RF”) communication applications as low noise parametric amplifiers, harmonic frequency generators, frequency controllers such as voltage-controlled oscillators, filters and tunable high impedance surfaces. Various devices would benefit from tunable capacitors or varactors having a wide tuning range, low loss and a low power consumption. A useful varactor should also be capable of being monolithically integrated in the device in which it is employed.
Typically, solid-state varactors are employed where tunable capacitance is required. However, solid-state varactors provide a very limited tuning range, usually in the range of 300-400%, have a high resistive loss and relatively high power consumption. In a solid state varactor diode used to provide a tunable capacitor, the varactor's capacitance is set by a bias current generated by a sub-circuit that can consume a significant amount of steady state power. Furthermore, the signal current applied to a solid-state varactor may tend to affect the capacitance, thus inducing some measure of error.
To avoid these problems, a MEMS varactor may be used in the place of solid-state varactors. MEMS devices are the microscopic equivalent of air-spaced variable/switchable varactors, and can be integrated into silicon chips using conventional wafer fabrication processes. Compared to varactor diodes, these devices are amenable to monolithic integration in a standard electronic circuit process without sacrificing performance.
The materials and fabrication techniques used to create a MEMS device include standard integrated circuit manufacturing materials and techniques. For example, MEMS devices may be fabricated by the under-etching of an area of a silicon chip's top metal layer to create a microscopic metal beam. Specifically, the MEMS varactor structures can be fabricated using a silicon on insulator (“SOI”) technology in which the top silicon is etched to define the varactor structures, a deposition layer is patterned to form a mechanical coupler, and then subjected to a front side hydrofluoric acid etch to partially remove the silicon dioxide to form the suspended varactor structure.
Alternately, the MEMS varactor structures can be fabricated using a higher performance but more expensive SOI technology in which the top silicon, silicon dioxide or a deposited insulator layer is etched to define the mechanical coupler and the backside is dry etched to partially remove the substrate to suspend the mechanical coupler and the two electrically isolated capacitor plates. These and other techniques of fabrication will be known to those skilled in the art.
Previous RF MEMS tunable varactors have employed a gap tuning design, wherein the two plates of the varactor are suspended apart from one another, for example on the aforementioned microscopic metal beam and on the base of the device. A set of actuation electrodes pulls one plate closer to the other plate by deforming this microscopic metal beam through electrostatic attraction, thus changing the capacitance provided by these plates.
One of the forces intrinsic to MEMS variable varactors having an electrostatic attraction mechanism is the restoring force. The restoring force is a mechanical force that tends to return the cantilever assembly to its initial or rest position, i.e., the position of the cantilever assembly with no voltage potential across the actuation electrodes. When the voltage potential across the actuation electrodes is lessened or removed, the restoring force causes the cantilever assembly displacement to decrease as the cantilever assembly returns to its initial position, thereby varying the capacitance of the MEMS varactor.
However, known MEMS varactors suffer from the setback of a small tuning range (<3:1) due to a phenomenon known as the “snap down” effect. This effect causes the gap between the two plates of the varactor to close abruptly as the electrostatic attraction force provided by a pair of actuation electrodes exceeds the spring restoring force. As such, prior art devices have had to avoid bringing the two plates of the varactor within a minimum distance to prevent this effect, thus reducing the MEMS varactor's overall tuning range. Typically, prior art MEMS varactors have been limited to a tunable range defined by a deflection of no more than one third of the distance between the parallel plates. These varactors employ a region of small deflection of the microscopic metal beam or other device on which one of the varactor's plates is mounted resulting in a continuously variable capacitance-versus-voltage characteristic for the varactor.
Once spacing is decreased by more than one third, the snap down phenomenon takes effect and the formerly free end of the microscopic metal beam makes contact with the base of the device. In prior art devices, this has happened and the device is in snap down mode, it is assumed that the maximum capacitance has been reached for the device. Because of the snap down effect, prior art MEMS varactors have often been employed as bi-stable devices, rather than as true varactors continuously tunable over a full range of capacitances.
An explanation of the snap down effect and its limit on the tuning ranges of conventional varactors is included in the description of a MEMS varactor developed by Young and Boser in “A Micromachined Variable Capacitor for Monolithic Low-Noise VCOS,” Tech. Digest of Solid State Sensors and Actuator Workshop, Hilton Head, S.C., Jun. 2-6, 1996, pp. 124-127. Another example of a conventional MEMS tunable varactor is disclosed by Change et al. in U.S. Pat. No. 5,959,516. Other known MEMS varactors exist offering a slightly improved tuning range (3-4:1), but at the cost of greatly increased power consumption. What is needed is an RF MEMS tunable varactor with a tuning ratio of over 40:1 which maintains the feature of ultra low power consumption (<<1 mW).